subsoil loosening does little to enhance the transition to no-tillage on a structurally degraded...

Soil & Tillage Research 68 (2002) 109–119

Subsoil loosening does little to enhance the transitionto no-tillage on a structurally degraded soil

M. Hamilton-Mannsa,∗, C.W. Rossb, D.J. Hornec, C.J. Bakerda Monsanto New Zealand Ltd., P.O. Box 36-562, Christchurch, New Zealandb Landcare Research, Private Bag 11-052, Palmerston North, New Zealand

c Institute of Natural Resources, Massey University, Private Bag 11-222, Palmerston North, New Zealandd Centre for International No-Tillage Research and Engineering, RD 5, Feilding, New Zealand

Received 11 July 2001; received in revised form 19 August 2002; accepted 9 September 2002

Abstract

Long-term cropping with conventional cultivation on New Zealand’s easily compacted soil causes soil structure degradation.The objective of this study was to ascertain if soil physical properties, and crop establishment and yield could be improved bysubsoil loosening in the first year of conversion from conventional tillage to no-tillage. Plots on a Milson silt loam (ArgillicPerch-Gley Pallic Soil, Typic Ochraqualf) were Paraplowed (PP), straight-legged subsoiler (SL), mole ploughed (M), or leftas non-loosened controls (C) in the autumn of 1997. Forage brassica was sown with a Cross-SlotTM no-tillage drill. Wheatwas established on the plots in the spring of 1997. Subsoil loosening resulted in some transient improvements in measuredsoil physical properties. Initially, subsoil loosening significantly reduced soil strength. Shortly after subsoil loosening, coneindices showed disruption to 300 mm with PP, 350 mm with SL and 100 mm with M. About 80% of profile cone indices fromthe PP and SL treatments were less than the critical value of 2 MPa compared to 48% for C and M. At 267 days after subsoilloosening, PP continued to have significantly lower cone index values than C and M. In May, the bulk density of PP plots wassignificantly lower than SL, M and C although reconsolidation in all plots was observed 9 months later after the wheat washarvested. Air permeability for PP, SL and M was significantly greater than C in June. Subsoil loosening did not increase plantpopulations or yield of the brassica or wheat crops. Vertical rooting depth was greater in the PP treatment. Few significantdifferences in wheat rooting patterns were found at depth.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:No-tillage; Subsoil loosening; Cross-SlotTM drill opener; Paraplow (PP); Straight-legged subsoiler (SL); Mole plough (M);Manawatu; New Zealand

1. Introduction

Tillage with mouldboard ploughs and subsequentsecondary cultivations has been the traditional methodfor New Zealand farmers to establish crops or pas-

∗ Corresponding author. Tel.:+3-337-5767; fax:+3-337-5768.E-mail address:[email protected](M. Hamilton-Manns).

ture. Such tillage relies on repeated passes with tynedand/or powered machinery to create a seedbed suitablefor crop establishment. In the cultivation processes,soil aggregates are disintegrated—not shattered andre-arranged along natural lines of cleavage, as theywould normally be through natural processes (Bakeret al., 1996). Repeated cultivations result in soil struc-tural decline, which causes the soil to compact overthe growing season, and the development of tillage

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110 M. Hamilton-Manns et al. / Soil & Tillage Research 68 (2002) 109–119

pans. Soil structural degradation and pan formationunder cultivated cropping has been widely reported inNew Zealand (e.g.Greenwood and Cameron, 1990;Harrison et al., 1994; Sojka et al., 1997; Shepherdet al., 2001).

Recent developments in machinery, herbicides andmanagement have re-established no-tillage as a viablealternative method for sowing crops, especially whenstarting with good soil conditions. However,Bakeret al. (1996)reported that smaller root systems, andreduced crop vigour and yield, have been observed un-der adverse soil conditions with no-tillage when com-pared with crops established by tillage. Thus, in soilswhere structure has been degraded as a consequenceof repeated and/or untimely cultivation, no-tillage maybe less successful than tillage. In this case, short-termamelioration of soil structure may be necessary un-til the natural processes for regenerating soil structurecan predominate.

Regenerative processes are encouraged by no-tillage(Baker et al., 1996) but the difficulty for farm-ers is through the transition phase from a tillageregime to a no-tillage system, especially whenstarting from a degraded soil structural condition.Some authors (e.g.Greenwood and Cameron, 1990;Harrison et al., 1994; Evans et al., 1996) have il-lustrated the need for deep loosening to alleviatecompaction and improve the agronomic performanceof crops or pastures established without cultivation.This observation is especially pertinent in finely tex-tured soils that have been subject to intensive tillagecropping.

Increased awareness of the problems associatedwith subsoil compaction has generated widespread in-terest in subsoiling. This technique has been reportedto provide benefits in soil physical properties (Swain,1975). The longevity of beneficial soil loosening bysubsoiling is variable, commonly 3–4 years (Ide et al.,1987; Twomlow et al., 1994; Carter and Kunelius,1998) and up to 5–6 years duration (Fukunaka, 1980).However, a number of papers have shown that sub-soiling effects are often short-lived, only lasting forone cropping season (e.g.Busscher et al., 1986; Meadand Chan, 1988; Porro and Cassel, 1986; Evans et al.,1996; Willis et al., 1997). Soils in poor structuralcondition are especially susceptible to rapid naturalreconsolidation, thus diminishing the longevity of thebenefits of subsoiling.

A range of subsoiling implements is available forcommercial use including straight-legged subsoilers(SLs), slant-legged subsoilers and mole ploughs (M).While all three implement types provide some soilloosening and shattering, M have the primary functionof drainage and are commonly used in conjunctionwith subsurface pipe or tile drainage systems. Authorsincluding Greenwood and Cameron (1990), Harrisonet al. (1994)and Evans et al. (1996)have reportedsubsoiling effects under tilled treatments but none havepreviously concentrated solely on no-tillage.

This paper describes a study of the effects of com-bining non-inversion cultivations or mole drainagewith Cross-SlotTM no-tillage seeding. The field ex-perimentation was conducted on a paddock where theimperfectly drained soil had become structurally de-graded from a period of previous mixed cropping us-ing conventional cultivation. Under a more traditionalcrop rotation regime, the paddock would normallyhave been “retired” for a period to restorative pasture.Instead, mixed cropping was continued through theconversion to a no-tillage system.

This study extended previous investigations (Sojkaet al., 1997) by applying combinations of subsoil-ing and mole drainage with no-tillage with twotest crops in succession—a winter forage crop(Brassica campestis× Brassica napuscv “Pasja”) andspring-sown wheat (Triticum aestivumcv “Kohika”).The Sojka et al. (1997)study was restricted to awinter oats (Avena sativacv “Awapun”) forage crop.

The objectives of the study was to ascertain if soilphysical properties, and crop establishment and yieldcould be improved by subsoil loosening in the first yearof conversion from conventional tillage to no-tillage.

2. Materials and methods

2.1. Field design and layout

The field experiment was located on a farm in theManawatu region of New Zealand (13 km northeastof Palmerston North, latitude S401/5238, longitudeE175/38143). The Manawatu plains support a rangeof land uses including intensive mixed livestock andcropping farms. The region is classified as “moisttemperate” (Brougham, 1979) with an average airtemperature of 17◦C. The soil at the experimental

M. Hamilton-Manns et al. / Soil & Tillage Research 68 (2002) 109–119 111

site is a Milson silt loam, a Gleyic Luvisol (ArgillicPerch-Gley Pallic Soil) (Hewitt, 1998), or a TypicOchraqualf (Soil Survey Staff, 1975). This soil ispoorly drained, exhibiting clay-rich, gleyed subsoils.The site has a 17 years history of mixed cropping.No-tillage began at the site in 1996.

In mid-February 1997, four treatments were estab-lished on a series of plots (8 m×40 m) in a field fromwhich a crop of no-tillage peas had recently been har-vested. Treatments were: M, SL, Paraplow® (PP), andcontrol (C). The M had a single straight shank operat-ing to 0.5 m. The Paraplow®, had two laterally angledshanks with a nominal operating depth of 0.45 m, andthe SL had five shanks designed to operate at 0.25 mdeep. Effective spacing between the subsoiler shanksfor PP and SL was 0.5 m. The M was operated at 2.0 mlateral spacing to a depth of 0.45 m. There was a tiledrain along the edge of the plots. All soil looseningoperations (40 m in length) were performed at rightangles to this drain.

The forage brassica crop (i.e. Pasja) was sown at3 kg ha−1 on 24 February 1997 with 200 kg ha−1 of(N–P–K) 8–15–15 fertiliser. When grazed to com-pletion, the forage crop was sprayed with Roundup®

herbicide (3.0 l ha−1 and 0.36 kg ai per litre). Springwheat was sown at 160 kg ha−1 on 30 October 1997with 250 kg ha−1 of (N–P–K) 15–10–10 fertiliser. Amolluscicide, “Slug Out” (18.0 g kg−1 metaldehyde)was broadcast at 8 kg ha−1. The four treatments weredrilled using a Cross-SlotTM no-tillage drill.

The experiment used a randomised complete blockdesign with four treatments and four replicates (i.e. 16plots). Measured parameters were evaluated statisti-cally employing SAS® software to determine standarddeviations, cumulative frequencies or probabilities oftreatment significance using analysis of variance, asappropriate.

2.2. Soil physical properties

Soil strength was assessed by penetration resis-tance cone index values. Cone indices were moni-tored using the Bush® recording penetrometer with astandard ASAE 30◦ 12.83 mm cone. Readings weretaken at 100 mm intervals on a 2 m lateral transect atright angles to the direction of subsoiling and moling.Readings down the soil profile were at depths of 10,30, 50, 70, 90, 110, 130, 180, 230, 280, 330, 380, 430

and 480 mm, giving a total of 294 cone index valuesfor each cross-sectional profile. This procedure wasreplicated twice per plot at randomly selected sites.Soil strengths were determined three times in 1997 on20 February (3 days after subsoiling and moling), 30June and 11 November (12 days after the wheat cropwas sown). On each occasion, gravimetric soil mois-ture contents were measured. Samples were bulkedaccording to treatment.

Interpretation of soil strength was through exami-nation of cone index cumulative frequency plots andtwo-dimensional plots of cone index isopleths. Ef-fects of profile soil water content changes on coneindex were evident. The penetration resistances arepresented as they occurred (and plant roots wouldhave experienced) in the field under the prevailingsoil water content profiles at the time of measurement.They have not been normalised for water content.

The volumetric soil water content over the rangeof disturbance patterns created by the soil looseningtreatments was monitored with Time Domain Reflec-tometry equipment (Topp and Davis, 1985). Soil vol-umetric water content was measured at regular inter-vals from mid-July until the end of August, which iswhen the soil is wettest. Three lengths of wave-guidewere used (150, 300 and 450 mm): measurement ofvolumetric soil water content for the three depths wasreplicated twice per plot.

Soil bulk density was measured using the coretechnique. On 10 May, soil samples were taken atthree depths: 0–150, 150–300 and 300–450 mm, us-ing cylindrical aluminium cores measuring 75 mm inlength and 100 mm in diameter. Holes were dug ran-domly in the plots and cores inserted in the middleof the three depth zones before being extracted. Thisprocess was replicated twice per plot. A Troxler®

Model 3440 (Anon., 1991) moisture/density gaugewas used as an alternative, non-destructive means ofmeasuring soil bulk density. Bulk density was mea-sured within the 0–300 mm depth on 12 May andagain on 28 February. Ten replicates were performedper plot at randomly chosen locations.

Readings of air permeability were taken with a“falling-head” permeameter (Hillel, 1980) across allplots on 15 August. Ten measurements were takenwithin each plot at randomly selected locations. Ateach location the pressure probe was inserted into thesoil to its operating depth of 50 mm. The elapsed time


for the pressure in the cylinder to fall by 20 kPa wasmeasured with a stopwatch and later converted to airpermeability.

2.3. Plant measurements

Forage crop establishment was measured on 23March 1997 (27 days after sowing) by countingthe number of plants in a 1 m length of drill row.This process was replicated at 10 sites chosen ran-domly within each plot. Wheat establishment wasmeasured on 30 November 1997 (30 days after sow-ing) using the same technique. Forage crop yieldwas measured by cutting, drying (80◦C) and weigh-ing above-ground dry matter contained within an(0.63 m2) exclusion cage. On 27 February 1998, thecereal plots were harvested at 14% moisture and yieldcalculated.

To measure Pasja root depth, profile pits measuring1 m wide and 300 mm deep were dug at right anglesto the direction of soil loosening operations. A freshprofile wall and plant roots were exposed by usingfine water spray to wash a 5 mm layer of soil away.Maximum rooting depth was measured in situ. Thisprocess was replicated at two sites chosen randomlywithin each plot. Blocks of soil were then carefullyremoved so as not to break the roots before beingwashed to remove excess soil before drying (80◦C)and weighing.

Wheat rooting patterns were measured immediatelyfollowing harvest. Two cores were sampled per plotcentred over wheat plants, to a maximum depth of 1 musing Giddings® drilling equipment. The 64 mm di-ameter cores were divided into 100 mm lengths andstored at 4◦C before washing. Subsamples 100 mmlong were separately washed and sieved in a root wash-ing apparatus. The dry weight (65◦C) of roots for eachdepth sample was measured. Total dry weight of rootsper core and proportions of roots (by density) in each100 mm depth zone were calculated.

3. Results and discussion

In 1997, winter was exceptionally dry and mild inthe Manawatu region. Rainfall during May–July 1997was only 48% of the long-term average for this pe-riod. Mean daily air temperatures for 1997 were simi-

lar to the 30 years mean, however, daily temperaturesfor May–July were approximately 19% warmer thanaverage.

Soil conditions at the time of soil loosening werevery dry. The soil surface of the plots subsoiled withthe Paraplow® and the SL was very uneven due to thesubstantial soil disturbance that occurred during theseloosening operations.

3.1. Soil volumetric moisture content

There were no consistent differences in soil mois-ture content between treatments (Table 1). Through-out the sampling period soil moisture was generallyclose to field capacity, as measured by samples froma nearby site (Wilde and Ross, 1996).

3.2. Bulk density

Bulk density in the 0–150 mm depth, as measuredfrom cores was 8%(P ≤ 0.05) greater in PP, SL and Ccompared with M (Table 2). The low value measuredfor M is surprising. The M treatments did, however,have a large degree of variation in bulk density valuesas reflected in the high standard deviations. There wereno significant differences between the PL, SL and Ctreatments for bulk density in the 0–150 mm depth.There were no differences between any of the treat-ments in either the 150–300 or 300–450 mm depths.Bulk density, as measured by the core technique, is arelatively insensitive measure of soil loosening effects(Carter et al., 1996; Evans et al., 1996).

Bulk density data obtained with the Troxler® den-sity probe revealed that in the 0–300 mm range; SLand PP had significantly(P ≤ 0.05) lower bulk den-sities than either M or C. At this depth, PP had asignificantly(P ≤ 0.05) lower bulk density than anyother treatment (Table 3), averaging 6% less than Mand C, and 2% less than SL. This possibly reflectsthe greater amount of soil loosening achieved with theParaplow® compared with the other treatments. Al-though these differences seems to be small, the valuesgiven inTable 3are the mean values of a large depth in-crement (0–300 mm). There are probably larger differ-ences between treatments within particular segmentsof the 0–300 mm zone.

The relatively high standard deviations for bulkdensity data measured for M using core procedure


Table 1Soil volumetric moisture content (%) over three depths (0–150, 150–300 and 300–450 mm) for the subsoil loosening treatments over wintera

Sample data

13 July 26 July 3 August 10 August 24 August 31 August

Volumetric moisture content at 0–150 mmC 39.0a 41.4a 43.9a 38.8a 37.6a 40.3aPP 43.9a 36.1a 43.9a 37.8a 35.2a 42.2aSL 43.1a 35.6a 42.9a 37.0a 36.1a 44.6aM 43.2a 40.7a 29.7b 37.8a 37.5a 36.5a

Volumetric moisture content at 150–300 mmC 45.9b 41.1b 29.1c 35.4b 40.6a 35.2dPP 45.4b 35.1c 43.4a 36.1b 38.3ab 42.9bSL 48.5a 44.9a 44.5a 43.1a 39.9a 38.7cM 50.5a 42.4a 34.6b 35.6b 37.4ab 49.6a

Volumetric moisture content at 300–450 mmC 49.1a 44.3a 42.8a 51.0a 44.8a 33.9aPP 43.0a 40.4a 40.8a 48.8a 45.9a 35.1aSL 46.9a 42.7a 46.9a 46.0a 47.8a 35.3aM 53.0a 47.4a 49.6a 45.0a 48.0a 40.0a

a Different letters within column or row denote significant differences at(P ≤ 0.05) within sampling dates.

Table 2Dry bulk density (g cm−3) measured by core technique on 10 May 1997 at three soil depths for subsoil loosening treatments (mean±standarddeviation)a

Soil depth (mm) Control (C) Paraplow (PP) Straight-legged subsoiler (SL) Mole plough (M)

0–150 1.23b± 0.08 1.27b± 0.06 1.24b± 0.09 1.15c± 0.22150–300 1.41a± 0.04 1.40a± 0.06 1.43a± 0.05 1.39a± 0.21300–450 1.44a± 0.02 1.45a± 0.05 1.44a± 0.02 1.43a± 0.06

a Different letters within a column or row denote significant differences at(P ≤ 0.05).

Table 3Wet bulk density (g cm−3) measured by the Troxler® moisture–density gauge on 12 May 1997 and 28 February 1998 for subsoil looseningtreatments (mean± standard deviation)a

Soil depth (mm) Control (C) Paraplow (PP) Straight-legged subsoiler (SL) Mole plough (M)

May 1997 1.35a± 0.01 1.26c± 0.02 1.29b± 0.01 1.34c± 0.08February 1998 1.36a± 0.015 1.33a± 0.018 1.32a± 0.019 1.35a± 0.017

a Different letters within a column or row denote significant differences at(P ≤ 0.05).

was also observed in the data measured with theTroxler® probe. This is probably due to the compre-hensive but very localised fracturing pattern createdby mole ploughing (Culpin, 1992). The Troxler®

method enabled both the standard deviation and LSDto be reduced by, on average, a factor of three forall treatments. This is consistent with the results ofHarrison et al. (1994).

The soil loosening achieved on PP, SL and Mplots, as measured by decreases in bulk density inMay was not apparent after cereal harvest the fol-lowing summer. There were no differences in bulkdensity between treatments at this time(P ≤ 0.05),all treatments having undergone some consolida-tion since May. The compactive effects of grazinglivestock and vehicular traffic may have been partly


responsible for the increase of bulk density valuesover time.

3.3. Cone index profiles

Four days before Pasja sowing, PP only had sig-nificantly lower cone index values than C through alldepths (Fig. 1a). The critical value of 2 MPa was ex-perienced within 70 mm depth for M and C, whereasit was reached at a depth of about 180 mm for PP andSL.

Soil strength measurements in June showed that PPcontinued to have lower cone index values than M andC (Fig. 1b). Soil moisture contents increased from anaverage of 11 to 33% by weight from the Februarymeasurements. Despite the increases in soil moisture,cone index values for M and C still reached 2 MPa nearthe surface (50 and 170 mm respectively) whereas PP

Fig. 1. Cone index profiles for the three sampling dates (bars are LSD atP ≤ 0.05): (a) 20 February; (b) 30 June; (c) 11 November. PP:Paraplow, SL: straight-legged subsoiler, M: mole plough, C: control.

and SL plots achieved this value at a depth approach-ing 370 and 330 mm, respectively. In February, moleploughing resulted in a reduction of cone index valuesto a depth of 100 mm (Fig. 1b) compared with C. Thepassages of the M were apparent with their surround-ing pattern of lateral soil disturbance.

In November, PP and SL had significantly lowervalues than C throughout most of the depth range andespecially in the 90–230 mm depth (Fig. 1c). All plotshad a greater incidence of cone values above 2 MPaand clearly displayed the effects of reconsolidation.This probably resulted from animal treading and ve-hicular traffic as the paddock had been intensivelygrazed over the winter, then sprayed and the wheatcrop established. A drier soil condition was also a fac-tor causing higher soil strengths.

The persistence of loosening for the PP treatmentswas obvious throughout the trial. This result concurs


with Sojka et al. (1997)who also found no advan-tage in terms of initial soil loosening to one particularsubsoiling implement but reported that the Paraplow®

plots showed a greater persistence of profile looseningthan the other treatments.

3.4. Cone index profile isopleths

Cone index profile isopleths reveal the spatial dis-tribution of subsurface soil disruption associated witheach treatment. At 20 February the C plots had themost extensive area of high cone index values nearthe surface of the profile (Fig. 2a). Disruption patternsfor February showed SL had the deepest (350 mm)overall loosening and increased disruption associatedwith shank placement was evident. Patterns of low soilstrength (0–1 MPa) were obvious to a depth of about250 mm in PP. Shank lines were associated with lowsoil strengths.

On 30 June, cone index values for C decreased,in response to increased soil moisture. Shank linesof SL, with the surrounding zones of loosened soil,remained clearly visible (Fig. 2b) although few coneindex values were in the 0–1 MPa range. Shanklines for PP, with surrounding zones of loosenedsoil, were still apparent in the disruption pattern andPP had the most extensive lateral disruption. Shanklines of M still showed as areas of lower strength,however, the effect was not as obvious as for PPand SL.

By November, cone index values for C once againincreased, with corresponding decreases in soil mois-ture (Fig. 2c). There were no distinctive features forSL evident at this time. Shank lines for PP remainedevident although consolidation had occurred with coneindex values exceeding 2 MPa below about 250 mmdeep. By November the shank lines on M plots were nolonger apparent, most cone index values being greaterthan 2 MPa.

3.5. Air permeability

Air permeabilities, measured in August in PP, SLand M plots were significantly greater than thosemeasured in C (data not shown). The average airpermeability for the PP, SL and M treatments was4.13 × 10−11 cm2 compared with 2.74 × 10−11 cm2

for C. The air permeability of soil depends on the

air-filled porosity of the soil and on the sizes andcontinuity of the soil pores. Thus air permeabilityresults indicate differences in soil porosity condi-tions between the treatments. Soil porosity was notspecifically measured in this trial.

3.6. Pasja population and yield

Subsoiling and mole drainage did not significantlyimprove the brassica (Pasja) crop establishment com-pared with the C. Plant populations for the treatmentswere: 83 plants m−2 for C; 68 plants m−2 for PP;80 plants m−2 for SL; 78 plants m−2 for M (LSD =16.8). Plant populations on PP plots were very vari-able. Poor plant establishment on PP plots comparedto the other treatments is explained by drier surfacesoil conditions at sowing. Surface soil on PP treat-ments was thoroughly loosened with no consolidationbefore drilling except for “wheeled” areas.

In a similar study,Sojka et al. (1997)also failed tomeasure a response in plant establishment to subsoil-ing in combination with no-tillage. Germination andestablishment under no-tillage are largely a functionof the type of opener used (Baker et al., 1996). Theopener used in this particular study, and that ofSojkaet al. (1997), has proved effective at minimising theinfluence of unfavourable soil (Choudhary and Baker,1981a) and micro-climate factors (Choudhary andBaker, 1981b) which might otherwise reduce standpopulation. Thus it is likely that the Cross-SlotTM

opener mitigated the effect of differences in bulkdensity and soil structure between treatments on plantestablishment. However, the opener was not able tototally overcome dryness problems associated withexcessively loose soil on PP plots.

There was no significant difference in brassicayield between treatments after 21 days regrowth(P ≤ 0.05). Plant yields were: 1786 kg DM ha−1 forC; 1925 kg DM ha−1 for PP; 1886 kg DM ha−1 forSL; 1807 kg DM ha−1 for M (LSD = 222). The cropgrew well at the trial site—the mean daily growthrate (88 kg DM ha−1) compared very favourably withthe reported daily winter growth rates for this crop(80 kg DM ha−1) (Anon., 1996). This is a reflection ofthe good growing conditions associated with the mild,dry winter. Linear regression analysis determined astrong negative relationship between plant populationand crop yield(r2 = 0.83) (P ≤ 0.05).


Fig. 2. Cone index isopleths for soil strength measurements on: (a) 20 February 1997 (top left); (b) 30 June 1997 (top right); (c) 11November 1997 (bottom). PP: Paraplow, SL: straight-legged subsoiler, M: mole plough, C: control.

A range of crop responses to subsoiling has beenreported in the literature, but almost all of those stud-ies were under conventional tillage regimes.Sojkaet al. (1997) found that winter forage oat yields

(sown by no-tillage) from autumn Paraplow® treat-ments yielded 30% greater than the C in soil that hadbeen continuously cropped for 15 years and had anextremely degraded structure.


Table 4Root length and root mass of forage brassica under subsoil loos-ening treatmentsa

Root length(mm)

Root weight(g)

Control (C) 127.0c 1.96abParaplow (PP) 157.7a 1.88abStraight-legged subsoiler (SL) 130.7c 1.99aMole plough (M) 134.8b 1.75b

a Different letters within a column or row denote significantdifferences at(P ≤ 0.05).

3.7. Pasja root depth and mass

PP had the greatest maximum rooting depth ofany treatment (Table 4). Maximum vertical rootingdepth in PP was an average 24% longer than both Cand SL, which together had the shortest root lengthof the four treatments. M had a significantly greatermaximum vertical rooting depth than SL and C. Thedifference in vertical rooting depth experienced in PPmay have reflected the deep (450 mm) soil looseningcaused by the Paraplow®. There were no significantdifferences in total root mass between PP, SL andC (Table 4). Unfortunately there are no known stud-ies that have measured the rooting characteristics of“Pasja”. In most experiments involving the looseningof compacted subsoils, subsoiling has increased rootgrowth (Chapman and Allbrook, 1987; Greenwoodand Cameron, 1990; Unger and Kaspar, 1994).

There are a number of factors likely to have con-tributed to the smaller percentage increase in rootgrowth after subsoiling and moling measured in thepresent study compared with others in the literature.The most probable reason is the crop type. The bras-sica used (“Pasja”) is characterised by large amountsof leaf and little bulb (Anon., 1996). In comparison,other experiments have used crops with more exten-sive and deeper root systems.

The significant increase in maximum vertical root-ing depth in PP did not translate into increased cropyields in this study. This is not surprising given thatthe crop grew through one of the driest and mildestwinters on record.

3.8. Wheat plant population and yield

There were no significant differences in wheat plantpopulations between treatments(P ≤ 0.05). Plant

populations for the treatments were: 240 plants m−2

for C; 250 plants m−2 for PP; 260 plants m−2 for SL;250 plants m−2 for M (LSD = 40). Damage by slugs(Deroceras reticulatum) resulted in actual plant popu-lations in all treatments below those targeted at sowing(i.e. 350 plants m−2).

There were no significant differences in final wheatyield between treatments. At an average of about5000 kg ha−1, the final yield was representative ofwheat harvested in the Manawatu in 1998. The dif-ferences in soil penetration resistance (Fig. 1) did nottranslate into yield differences in this study. As thesummer of 1998 was hot and dry, the lack of responseto PP cannot be attributed to moist summer climaticconditions, however, the lack of a subsoiling responsein cereal production is not uncommon. For example,Evans et al. (1996)concluded that a single subsoilingoperation had very little effect on plant growth andno effect on grain yield over the following seasons.Likewise,Carter et al. (1996)indicated that increasesin crop productivity because of subsoiling are minor.

3.9. Wheat rooting patterns

There were no statistically significant differencesbetween any of the treatments for either the totalweight of roots or root density patterns (Fig. 3). Av-erage total dry weights of wheat roots per core were0.91, 0.92, 1.08 and 0.90 g for C, PP, SL and M,respectively.

Rooting patterns are given inFig. 3, where theproportion of roots on a density basis (g of root percm−3 of soil) is plotted against soil depth. The dataindicate somewhat deeper rooting under the M andPP treatments. Both subsoiling and moling resulted inslightly higher proportions of roots in the 50–80 cmzone. This finding is consistent with the literature(Ide et al., 1984; Unger and Kaspar, 1994; Harrisonet al., 1994) that generally reports deeper rooting re-sulting from deep tillage or drainage of compactedand gleyed subsoils. However, these differences aremostly in the 0.1–1.0% range and are thus minor inrelation to the total root system.

In contrast, a trend of slightly shallower rooting wasevident for SL (relatively more roots in the 0–40 cmzone). This suggests that the autumn loosening effectsof the SL were negated by the time the wheat crop wasplanted in the spring. This hypothesis is supported by


Fig. 3. Proportion of wheat root densities (g roots cm−3) down thesoil profile for C, PP, SL and M treatments after crop harvest.PP: Paraplow, SL: straight-legged subsoiler, M: mole plough, C:control.

the soil physical data. Also only small significant dif-ferences in rooting patterns are in accord with the soilphysical data, which indicates that the initial benefitsof the subsoiling and mole drainage were diminishedby the spring wheat crop. The mild, moist growingconditions in the spring and early summer probablylimited the need for wheat plants to develop deep root-ing patterns in the Milson soil, which has physicalsubsoil impediments. It was thus assumed that 90 cmis the approximate maximum depth of wheat root pen-etration in this soil. Over 80% of the roots were inthe 0–10 cm layer for all treatments, a finding consis-tent with that ofFrancis et al. (1987)for direct-drilledwheat in a loess soil.

Small differences in rooting patterns did not trans-late into differences in crop yields (see previous sec-tion) because of seasonal factors, as outlined above,and drill opener design.

Given that subsoiling significantly improved soilphysical conditions, soil loosening performed inspring, in combination with no-tillage, may be bene-ficial to the growth of the following crop, particularlyin a dry summer. The combination of subsoiling ormole drainage with no-tillage might be expected toproduce agronomic advantages when conditions are

either very dry or very wet and for other no-tillageopeners.

4. Conclusions

• Combining subsoiling or moling with Cross-SlotTM

no-tillage on a poorly drained, structurally degradedsoil had neither beneficial nor disadvantageous ef-fects on the following winter forage and springwheat crops.

• Soil loosening initially improved soil physical con-ditions in the 0–300 mm zone. This resulted inminor improvements to rooting patterns (deeper)for both Pasja and wheat crops. No significant dif-ferences in soil physical conditions between thetillage treatments and C were evident after thesummer crop. However, this did not translate intoimproved crop performance under the seasonalconditions encountered over the research period.

Acknowledgements

Special thanks to Ross and Helen Maxwell for al-lowing us to conduct the research on their farm, andfor providing considerable logistical support. Finan-cial support for part of the research was provided bythe Foundation for Research, Science and Technol-ogy under contract no. CO9402. Thanks to AdrianHamilton-Manns for his assistance and to John Dandofor his help in the field, laboratory and data process-ing work; to Joe Whitton for his assistance with rootwashing; and to Joanne Morris and Elaine Hagenaarsfor their graphics expertise.

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subsoil loosening does little to enhance the transition to no-tillage on a structurally degraded...

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